Hmmm. I am also dead on 6′ and ride two Spesh 56cm frames (Langster and Roubaix) I certainly wouldn’t want them any bigger.
The correct answer is to go and get properly measured and get the “right” size. However, unless you are oddly proportioned I would go for the 56. It’s easier to get a bit more length out of a bike if it’s slightly too small, than to try and make a bike which is too big fit you.